Mon. Not. R. Astron. Soc. 418, 2720–2729 (2011) doi:10.1111/j.1365-2966.2011.19661.x The Cosmic Background Imager 2 , Angela C. Taylor,1 Michael E. Jones,1 James R. Allison,1 2 Emmanouil Angelakis,3 J. Richard Bond,4 Leonardo Bronfman,5 Ricardo Bustos,5,6 Richard J. Davis,7 Clive Dickinson,7 Jamie Leech,1 Brian S. Mason,8 Steven T. Myers,9 Timothy J. Pearson,10 Anthony C. S. Readhead,10 Rodrigo Reeves,10,11 Martin C. Shepherd10 and Jonathan L. Sievers4 1Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH Downloaded from https://academic.oup.com/mnras/article/418/4/2720/1029397 by guest on 02 October 2021 2Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia 3Max-Planck-Institut fur¨ Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany 4Canadian Institute for Theoretical Astrophysics, University of Toronto, ON M5S 3H8, Canada 5Departamento de Astronom´ıa, Universidad de Chile, Casilla 36-D, Santiago, Chile 6Departamento de Astronom´ıa, Universidad de Concepcion,´ Casilla 160-C, Concepcion,´ Chile 7Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL 8National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 9National Radio Astronomy Observatory, Socorro, NM 87801, USA 10Cahill Center for Astronomy and Astrophysics, Mail Code 249-17, California Institute of Technology, Pasadena, CA 91125, USA 11Departamento de Ingenier´ıa Electrica,´ Universidad de Concepcion,´ Casilla 160-C, Concepcion,´ Chile Accepted 2011 August 18. Received 2011 August 18; in original form 2011 July 7 ABSTRACT We describe an upgrade to the Cosmic Background Imager instrument to increase its surface brightness sensitivity at small angular scales. The upgrade consisted of replacing the 13 0.9-m antennas with 1.4-m antennas incorporating a novel combination of design features, which provided excellent sidelobe and spillover performance for low manufacturing cost. Off-the- shelf spun primaries were used, and the secondary mirrors were oversized and shaped relative to a standard Cassegrain in order to provide an optimum compromise between aperture efficiency and low spillover lobes. Low-order distortions in the primary mirrors were compensated for by custom machining of the secondary mirrors. The secondaries were supported on a transparent dielectric foam cone to minimize scattering. The antennas were tested in the complete instrument, and the beam shape and spillover noise contributions were as expected. We demonstrate the performance of the telescope and the intercalibration with the previous system using observations of the Sunyaev–Zel’dovich effect in the cluster Abell 1689. The enhanced instrument has been used to study the cosmic microwave background, the Sunyaev– Zel’dovich effect and diffuse Galactic emission. Key words: instrumentation: interferometers – methods: data analysis – cosmic background radiation – X-rays: galaxies: clusters. ization (Padin et al. 2001; Mason et al. 2003; Pearson et al. 2003; 1 INTRODUCTION Sievers et al. 2003, 2007, 2009; Readhead et al. 2004). In addition, The Cosmic Background Imager (CBI; Padin et al. 2002) was a it was also used to make observations of the Sunyaev–Zel’dovich 13-element comounted interferometer operating at 26–36 GHz, de- (SZ) effect in a sample of low-redshift (z ≤ 0.1) clusters (Udom- signed primarily to observe the power spectrum of fluctuations in prasert et al. 2004), and measurements of ‘anomalous’ microwave the cosmic microwave background (CMB) on angular scales of emission from dust in a range of Galactic objects (Casassus et al. 5arcminto1◦ (multipoles ∼ 400 to ∼3500). Between 2000 Jan- 2004, 2006, 2008; Hales et al. 2004; Dickinson et al. 2006, 2007). uary and 2005 April, the CBI operated from the Chajnantor Plateau, These observations were made using antennas 90 cm in diam- Chile, at an altitude of 5100 m and during this period it made ob- eter. In 2005–2006, the CBI was upgraded to larger 1.4-m anten- servations of the CMB power spectrum in both intensity and polar- nas (‘CBI2’) to increase the effective collecting area and to allow observations at higher resolution without compromising surface brightness sensitivity. Observations with the CBI2 continued un- E-mail: [email protected] til 2008 June, after which its site and mount were used for the C 2011 The Authors Monthly Notices of the Royal Astronomical Society C 2011 RAS The Cosmic Background Imager 2 2721 QUIET experiment (QUIET Collaboration et al. 2010). During this i.e. corresponding to angular scales of 6–12 arcmin, on which the period, the CBI2 completed a programme of observations of dif- CBI1 array was not well filled. Improved sensitivity on these longer fuse Galactic emission, the CMB power spectrum and targeted SZ baselines would provide significantly improved observations of the clusters (Dickinson et al. 2009, 2010; Castellanos et al. 2011; Vidal SZ effect in massive galaxy clusters. In CBI1 SZ observations, et al. 2011, and further papers in preparation). In this paper, we the shortest baselines were heavily contaminated by primary CMB describe the antenna design that was used in the CBI2 upgrade. We anisotropies, while the longer baselines lacked thermal sensitivity. summarize the main science goals for the upgrade and present com- Moderately massive clusters typically have virial radii of ∼2Mpc, missioning results that confirm its effectiveness. We also present a which at a redshift of z ∼ 0.15 corresponds to an angular size of combined analysis of an SZ detection in the cluster A1689. This ∼12 arcmin. This is well matched to the new CBI2 array, which cluster was observed both with the original CBI (hereafter ‘CBI1’) is thus able to measure the cluster gas out to the outskirts of the and with the upgraded CBI2, and allows us to demonstrate both the clusters with significantly less contamination from primary CMB intercalibration of the two instruments and the benefit of measuring fluctuations than was the case for CBI1. the SZ decrement with the larger CBI2 antennas. The motivation to concentrate on measuring the SZ effect out Downloaded from https://academic.oup.com/mnras/article/418/4/2720/1029397 by guest on 02 October 2021 to the virial radius in complete samples of clusters was driven by the need to further understand the X-ray–SZ and weak-lensing–SZ 2 SCIENCE MOTIVATION scaling relations in support of SZ survey experiments. The SZ ef- 2 The angular scales to which an interferometer is sensitive are set fect measures the Comptonization parameter, y = kTe/(mec )dl, by the lengths of the baselines between the antennas, with longer which is proportional to the electron pressure integrated along the baselines responding to finer scale information in the sky brightness. line of sight. SZ surveys are designed to measure the integrated However, for a fixed antenna size, the sensitivity of a baseline to SZ effect, Y = y d, providing empirical measurements of the extended sources decreases rapidly as the baseline is lengthened. In cluster comoving SZ luminosity function dN/dY.However,inor- the Rayleigh–Jeans limit, the temperature sensitivity T is given der to relate these measurements to cosmology via the cluster approximately by mass function, dN/dM, a well-calibrated relationship between Y and the total mass M is required. This can be achieved by com- T = λ2S/ kf , (2 ) (1) bining SZ measurements of known clusters with X-ray and weak where S is the flux density (point source) sensitivity, is the lensing data, along with modelling that accurately describes the dis- solid angle of the main lobe of the synthesized beam, and the filling tribution of the cluster components (dark matter, gas and galaxies) factor, f , is the fraction of the synthesized aperture that is filled in a way that can be constrained by the observational data. There with antennas. This is simply a modification of the Rayleigh–Jeans have been a number of recent measurements of the scaling between equation to reflect the fraction of photons captured instantaneously the integrated SZ effect and the total mass, from both hydrostatic by the aperture – the exact temperature sensitivity as a function (Benson et al. 2004; Bonamente et al. 2008) and gravitational lens- of angular scale will depend on the configuration of the antennas ing (Marrone et al. 2009) estimates. However, these relationships within the synthesized aperture. Increasing the resolution of an have generally only been obtained out to relatively small radii interferometer without losing brightness sensitivity thus requires (∼200–400 kpc), and observations with experiments such as CBI2, that either the number of antennas be increased or the antenna APEX-SZ (Schwan et al. 2003) and AMIBA (Ho et al. 2009) are ∼ size be increased, in order to maintain the filled fraction of the expected to provide constraints out to a few Mpc ( r200,whererx synthesized aperture. If the number of baselines is fixed, and the denotes the radius within which the average density is x times the antennas are not changed, lengthening the baselines results in an critical density). increase in integration time to reach the same temperature sensitivity Fig. 1 illustrates why measurements at large angular scales rel- proportional to the fourth power of the baseline length. ative to the core of the cluster are important in determining true The primary goal of the CBI2 upgrade was to increase the temper- SZ profiles. It shows the thermal SZ effect for toy models of three ature sensitivity of the instrument on its longer baselines of 3–5.5 m, clusters at z = 0.15, with similar cores but different large-scale Figure 1. Left: the change in thermodynamic temperature relative to the CMB due to the thermal SZ effect from three clusters, at z = 0.15, with different large-scale properties.